This invention relates to a frame for a game racquet, and, more particularly, to a frame which is formed by filament winding and resin transfer molding.
The invention will be explained with reference to frames for tennis racquets. It will be understood, however, that the invention can be used to form frames for other game racquets, such as racquetball racquets, squash racquets, badminton racquets, etc.
Current racquet production methods take composite fiber and resin prepreg materials and roll, wrap, or fold the material by hand into a racquet preform shape. Some racquet frames have been formed by filament winding method in which a tow of resin wetted fiber or resin impregnated fiber (towpreg) is wound over a mandrel to form the preform. The preform is then inserted into a female tool and molded while pressure is applied through an internal bladder inflated with compressed air or by expanding polymeric foam. The methods of both hand layup and wet filament winding are labor intensive, require secondary operations prior to finishing (due to poor as-molded surface finish), and need improved properties for durability or impact strength for a game racquet application. The post-molding operations are labor-intensive and introduce a risk of decreased durability from oversanding the filler material. Wet filament winding uses conventional helical wound tow patterns that by design have inadequate shear properties due to winding discrete layers in the laminate.
Filament winding is a well known process for forming products from filament material such as graphite fiber, glass fiber, etc. In the filament winding process, filament material is wound around a rotating, mandrel to form a tube. The filament material can be in the form of a tow, which is formed from a plurality of filaments or continuous fibers, for example, with carbon 3000, 6000, or 12000 filaments per tow. Fewer or more filaments per tow can be used for carbon as well as other fibers, for example, from one to 50,000.
The filament material is coated with resin before or after winding. The wound tube is placed in a mold having the shape of the end product, and the mold is heated to cure the resin.
Filament wound parts are normally manufactured by winding discrete layers of material on a mandrel, using a helical or geodesic path. Each layer is made up of a 2-ply balanced laminate formed from a multicircuit winding pattern. The number of circuits required for each layer is calculated from the bandwidth of the material and the wind angle, so that the pattern closes and a constant thickness is achieved in the layer. Different layers may be wound at different angles, but each layer typically consists of a +.alpha..degree. angle ply and a -.alpha..degree. angle ply with respect to the longitudinal axis of the rotating mandrel, resulting from the reciprocating traverse of the carriage in the winding machine.
Conventional filament winding produces discrete layers or lamina of fibers. The layers have a tendency to move relative to each other when the wound article is stressed, during removal from the mandrel and loading of the wound tube in the mold. Also, the physical properties of the article are often limited by the shear properties between layers, called interlaminar shear.
Tennis rackets have been produced by conventional filament winding. For example, EPO patent publication No. 0 470 896 describes forming a frame for a tennis racket by winding fibers of glass, carbon, or other materials.
Preforms for racquet frames are generally made from fibers which have been wetted with resin. Sheets or ribbons of fiber and resin are referred to as prepreg. Tows of fiber and resin are referred to as towpreg. However, some attempts have been made to form a dry preform which is wetted with resin in the mold. For example, preforms have been made from braided or woven fibers. The braided or woven preform is inserted into a mold, and the preform is wetted by a resin transfer molding (RTM) or a reaction injection molding (RIM) method. In reaction injection molding a two part monomer system is used, and the monomers react and polymerize in the mold.
Previous attempts to use dry preform for making racquet frames have not been commercially significant, and it is believed that the preforms do not become completely and thoroughly wetted by the resin during the molding operation.
The following definitions are used herein:
CIRCUIT: One complete traverse of the fiber feed mechanism of a filament winding machine. PA1 INTERSPERSE: To sequence wind circuits from two or more patterns in an arbitrary manner as selected by the designer. Each circuit is appied individually such that any combination and sequence of wind angles from the patterns may by applied. No discrete layers or lamina exist. PA1 TOW: Untwisted bundle of continuous filaments PA1 PLY: A single pass on a filament winding machine in which fiber is applied in one direction to the axis of the mandrel PA1 A LAYER is typically formed from a series of winding circuits resulting in a closed pattern of 2-plies; one of +.alpha..degree. and the other of -.alpha..degree.. PA1 LAMINA: A single ply or layer in a laminate made up of a series of layers.